藏东碧土蛇绿岩对羌塘构造属性和羌塘油气勘察的启示

Bitu ophiolite in eastern Tibet:Indicator for Qiangtang tectonic affinity and petroleum and gas exploration

藏东北澜沧江构造带一直被认为是连接藏中龙木措—双湖缝合带和藏东南昌宁—孟连缝合带之间的桥梁,但蛇绿混杂岩带的缺失是制约这一认识的关键因素。近年来随着左贡碧土蛇绿岩带的发现,使南北羌塘地块在藏东地区的边界乃至与东南亚地区的块体对比成为现实。本文提供了碧土蛇绿岩套新的岩石学和年代学证据,新的辉长岩锆石U-Pb LA-ICP-MS谐和年龄在285.6±3.2 Ma左右,年龄数据表明碧土混杂岩带具有古特提斯缝合带属性。这可初步证明龙木措—双湖缝合带和昌宁—孟连缝合带可构成一条连续完整的古特提斯缝合带,并赞同前人关于南羌塘块体冈瓦纳属性及北羌塘华夏块体属性的观点。通过南北羌塘地块边界缝合带与周缘块体缝合带对比,进一步限定了新生代高原块体的挤出幅度很小,羌塘块体新生代陆内变形以挤压缩短和增厚隆升为特征。羌塘块体大幅度的挤压缩短表明羌塘油气勘查要取得突破必须加强深部勘探。

The North Langcangjiang tectonic belt is treated as joint between the Longmu Co-Shuanghu Paleotethyan suture and the Changning-Menglian suture as the boundary of the Cimmerian continents from the Laurasia. However, this proposal has been challenged by the absence of ophiolite mélanges in 700 km length in eastern Tibet. Here we provided new petrologic and chronological evidence for the Bitu mélange suite in eastern Tibet. Zircon U-Pb dating indicates that the gabbro crystallized at ~285.6±3.2 Ma, which attest that the Bitu suture is a Paleotethyan suture. Thus the Bitu suture, the Longmu Co-Shuanghu suture and the Changning-Menglian suture form an integral Paleotethyan suture zone, which divided the Qiangtang block into two parts. Blocks correlation of the South and North Qiangtang blocks with neighbor blocks constrain the Cenozoic deformation of the Qiangtang blocks with distributed deformation feature, which further indicates the petroleum and gas exploration should pay attention to deep crust structural trap.